A source of benzene and xylene is catalytic reformate, which is prepared by contacting a mixture of petroleum naphtha and hydrogen with a strong hydrogenation/dehydrogenation catalyst, such as platinum, on a moderately acidic support, such as a halogen-treated alumina. Usually, a C6 to C8 fraction is separated from the reformate and extracted with a solvent selective for aromatics or aliphatics to produce a mixture of aromatic compounds that is relatively free of aliphatics. This mixture of aromatic compounds usually contains benzene, toluene and xylenes (BTX), along with ethylbenzene.
Refineries have also focused on the production of benzene and xylene by transalkylation of C9+ aromatics and toluene over noble metal-containing zeolite catalysts. During the transalkylation of C9+ aromatics and toluene to high value petrochemical products, such as benzene and xylene, over catalysts containing noble metals, by-products, such as saturated compounds, are typically produced in the process. These by-products can boil in the same temperature range as the desired aromatic products, making separation of the desired products at high purity levels difficult. For example, a commercial benzene product may need a purity of 99.85 wt. % or higher. However, initial benzene purity after distillation of a transalkylation reaction product is typically only 99.2% to 99.5% due to the presence of coboilers, such as methylcyclopentane, cyclohexane, 2,3-dimethylpentane, dimethylcyclopentane and 3-methylhexane. Therefore, an additional extraction step is usually required to further improve benzene product purity to the desired level.
One solution to the problem of the production of benzene co-boilers during the transalkylation of heavy aromatics is disclosed in U.S. Pat. No. 5,942,651 and involves the steps of contacting a feed comprising C9+ aromatic hydrocarbons and toluene under transalkylation reaction conditions with a first catalyst composition comprising a zeolite having a constraint index ranging from 0.5 to 3, such as ZSM-12, and a hydrogenation component. The effluent resulting from the first contacting step is then contacted with a second catalyst composition which comprises a zeolite having a constraint index ranging from 3 to 12, such as ZSM-5, and which may be in a separate bed or a separate reactor from the first catalyst composition to produce a transalkylation reaction product comprising benzene and xylene. A benzene product having a purity of at least 99.85% may be obtained by distilling the benzene from the transalkylation reaction product, without the need for an additional extraction step. According to the '651 patent, the second catalyst composition comprises up to 20 wt. % of the total weight of the first and second catalyst compositions.
U.S. Pat. No. 5,905,051 discloses a process for converting a hydrocarbon stream such as, for example, a C9+ aromatic compound to C6 to C8 aromatic hydrocarbons, such as xylenes, by contacting the stream with a catalyst system comprising a first catalyst composition and a second catalyst composition, wherein said catalyst compositions are present in separate stages and are not physically mixed or blended and wherein said first catalyst composition is a metal-promoted, alumina- or silica-bound zeolite beta, and said second catalyst composition is ZSM-5 having incorporated therein an activity promoter selected from the group consisting of silicon, phosphorus, sulfur, and combinations thereof. According to the '051 patent, the use of the separate catalytic stages improves the conversion of C9+ aromatic compounds and naphthalenes to xylenes and decreases the amount of undesirable ethylbenzene in the product.
U.S. Pat. No. 5,030,787 discloses an improved disproportionation/transalkylation process. The improved process of this invention is conducted such that transalkylation of a C9+ aromatics feedstock, or disproportionation of a feedstock containing toluene and C9+ aromatic(s), is carried out in the vapor-phase by containing said feedstock in a reaction zone with a catalyst comprising a zeolite possessing a Constraint Index, as defined below, of from 1 to about 3 and preferably which has been hydrogen, hydrogen precursor and/or non-noble Group VIII metal exchanged, thermally treated and/or hydrothermally treated, under conditions effective to convert such feedstock to a product containing substantial quantities of C6-C8 aromatic compounds, e.g. benzene and xylene(s), especially the latter. The product effluent is separated and distilled to remove the desired products. If desired, any unreacted material(s), e.g., toluene and/or C9+ compound(s), can be recycled.
U.S. Pat. No. 5,030,787 discloses a transalkylation process to convert a heavy aromatics feed to lighter aromatics products, such as benzene, toluene and xylenes by contacting a C9+ aromatics fraction and benzene and/or toluene over a catalyst comprising a zeolite, such as ZSM-12, and a hydrogenation component, preferably platinum. The catalyst, with hydrogenation component, is treated to reduce aromatics loss. Treatment includes exposure to steam and/or sulfur after incorporation of the hydrogenation component. For additional stability and aromatics retention, the steamed and/or sulfur treated catalyst is sulfided by cofeeding a source of sulfur. In a further embodiment of the invention, a low hydrogen partial pressure is employed to retain aromatics.
U.S. Pat. No. 7,148,391 discloses a single stage catalyst system comprising at least two different molecular sieves that exhibits enhanced activity for the removal of ethyl-group containing aromatic compounds in C9+ aromatic feeds without overall reduction in the conversion of the C9+ feed to useful compounds, such as xylenes.
Improving catalytic activity and stability are challenges for most of the catalytic transalkylation processes. High activity catalyst normally requires less catalyst and/or less severe reaction conditions to manufacture the same amount of product, which means lower cost for production and high production efficiency. As the catalyst ages with increasing time on stream, higher temperatures are normally required to maintain constant conversion. When the maximum reactor temperature is reached, the catalyst needs to be replaced or regenerated. Depending on the feed composition, the cycle length varies from a few months to as long as a few years for a transalkylation catalyst. A catalyst having high stability normally requires less frequent regeneration or change-out and long time on stream, which translates to lower cost for production and high production efficiency.
The aging rate of catalysts used for the transalkylation of heavy aromatics is normally dependent on the nature of the feed composition. The higher the ratio of C9+ aromatics to C6-C7 aromatics, the greater the aging rate. In addition, the aging rate usually increases with an increasing concentration of material having C10+ aromatics, which are by-products of the transalkylation process. There are many chemical reactions that can lead to the formation of these heavier compounds, for example:Ethyl-methylbenzene+Ethyl-methylbenzene→Toluene+C11  (1)Ethyl-methylbenzene+Ethyl-methylbenzene→Ethylbenezene+C10  (2)Ethyl-methylbenzene+Trimethylbenzene→Toluene+C11  (3)Propylbenzene+Toluene→Benzene+C10  (4)Ethyl-dimethylbenzene+Ethyl-dimethylbenzene→Xylene+C12  (5)Ethyl-dimethylbenzene+Trimethylbenzene→Xylene+C11  (6)
There is, therefore, a need for a catalyst system to minimize the formation of these heavy aromatic compounds as they may be precursors for the formation of coke which reduces catalyst activity. One common feature of these reactions producing heavy aromatics is that most of them contain at least one reactant having an alkyl substituent with two or more carbon atoms, for example, an ethyl group or a propyl group. These molecules normally comprise a significant fraction of the feed to a transalkylation unit. Sometimes, ethyl-methylbenzenes and ethyl-dimethylbenzenes can comprise up to one third of the C9+ feed to the transalkylation unit. It has now been discovered that minimizing the reactions of these ethyl and propyl aromatics improves catalytic activity and/or aging rate.
In order to minimize these reactions of C10+ formation, it is preferable to dealkylate the ethyl and propyl groups from the aromatic molecules, and saturate the resulting olefin to prevent realkylation onto an aromatic ring. We surprisingly found that by dealkylating the ethyl and/or propyl groups in the feedstock, the formation of heavier aromatics, i.e., C10+ aromatics, is minimized, therefore reducing the catalyst aging rate. Not intended to be limited by any theory, we believe that it is desirable to de-alkylate the ethyl and propyl groups in the feed before undergoing transalkylation reactions. We found a catalyst system comprising a first catalyst that favors dealkylation over transalkylation reactions and a second catalyst that favors transalkylation over dealkylation reactions and the feedstock feeding to the first catalyst prior to the second catalyst. While not wishing to be bound by theory, we believe that transalkylation reactions take place via biphenylic-type transition states, and are favored in zeolitic catalysts having large channels, for example, in a 12 member-ring (12 MR) zeolites, e.g. mordenite, beta, ZSM-12, etc. Zeolites having 10 MR structures, for example ZSM-5 (MFI), tend to restrict the formation of this transition state necessary for transalkylation reactions, and therefore favor dealkylation reactions instead.
We, therefore, disclose a catalyst system for the transalkylation of C9+ aromatics with C6-C7 aromatics. The catalyst system comprises (a) a first catalyst comprising a molecular sieve having a Constraint Index in the range of 3-12 (e.g., a 10 MR molecular sieve, such as ZSM-5, ZSM-11, ZSM-22, and ZSM-23) and a metal catalyzing the saturation of the olefins formed by the dealkylation reactions and (b) a second catalyst comprising a molecular sieve having a Constraint Index in the range of less than 3 (e.g., a 12 MR molecular sieve, such as ZSM-12, MOR, zeolite beta, MCM-22 family molecular sieve) and optionally a metal which may be the same or different to the metal on the first catalyst.
We also surprisingly discovered the catalyst system and a new process of using the catalyst system for transalkylation reactions comprising contacting a C9+ feed with the first catalyst to form a product and then contacting at least a portion of the product with the second catalyst. This novel process allows for processing of heavy aromatic feed at high space velocities (high catalytic activity), which provides a significant advantage for a higher throughput transalkylation process. In addition, we surprisingly discovered that the use of this process and/or the catalyst system results in low aging rates for the catalyst system, thereby extending cycle lengths.